1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system for applying a gradient magnetic field and an RF pulse to an object to be examined which is placed in a static magnetic field so as to excite magnetic resonance at a specific portion of the object, and acquiring magnetic resonance (MR) echo signals excited by the magnetic resonance, thereby imaging the specific portion by a predetermined image reconstruction method using data based on the acquired MR echo signals and, more particularly, to an MRI system which allows an MR image with reduced chemical shift artifacts to be obtained by a hybrid echo method in which a spin echo method (to be referred to as an SE method hereinafter) using 90.degree.-180.degree. series RF pulses is improved to increase an S/N ratio.
2. Description of the Related Art
In a general medical MRI system, a gradient magnetic field and an RF pulse are applied to an object to be examined which is placed in a static magnetic field in accordance with a predetermined sequence for magnetic resonance excitation/MR data acquisition so as to cause an MR phenomenon at a specific portion of the object, and an MR signal excited by the MR phenomenon is detected. In addition, according to the system, data processing for imaging which includes image reconstruction is performed for MR data acquired in this manner so as to image anatomical information or quality information of the specific portion of the object.
An MRI system of this type generally comprises a static magnetic field generator, X-axis, Y-axis, and Z-axis gradient magnetic field generators, an RF transmitter, and an RF receiver. The X-axis, Y-axis, and Z-axis gradient magnetic field generators and the RF transmitter are driven in accordance with a predetermined sequence so as to generate X-axis, Y-axis, and Z-axis gradient magnetic fields Gx, Gy, and Gz and an RF pulse in accordance with a predetermined sequence pattern. As a result, magnetic resonance is excited to generate an MR signal, and the MR signal is received by the receiver. Predetermined image processing including image reconstruction processing is performed for the received MR data. In this manner, a tomographic image of a certain slice portion of an object to be examined is generated and displayed on a monitor.
In the sequence for magnetic resonance excitation/MR data acquisition, the X-axis, Y-axis, and Z-axis gradient magnetic fields Gx, Gy, and Gz are respectively used as, e.g., a read gradient magnetic field Gr, an encode gradient magnetic field Ge, and a slicing gradient magnetic field Gs.
One of the conventional MRI methods widely used in such a system is an imaging method employing the sequence of the SE method which uses 90.degree.-180.degree. series RF pulses.
The sequence of such a conventional SE method will be described below with reference to FIG. 1. FIG. 1 shows a sequence in one encode step.
A slicing gradient magnetic magnetic field Gs and a 90.degree. selective excitation pulse as an RF magnetic field are applied to an object to be examined so as to excite a specific slice of the object (to flip the magnetization vector (to be referred to as "nuclear magnetization" hereinafter) of the nuclear spin of a specific atomic nucleus in the slice through 90.degree.). Thereafter, an encode gradient magnetic field Ge having an amplitude corresponding to the encode step is applied to the object, and a 180.degree. pulse as an RF magnetic field is applied to the object so as to invert the nuclear magnetization, thereby rephasing and refocusing the rotational phase of the nuclear magnetization (which has been dephased and dispersed upon application of the 90.degree. pulse). In addition, a read gradient magnetic field Gr is applied to the object to generate a spin echo signal whose peak appears after a TE time (echo time) from the peak of the 90.degree. pulse. While the read gradient magnetic field Gr is applied to the object, the MR echo signal is acquired.
The above-described sequence is repeated while the amplitude of the encode gradient magnetic field Ge, which is applied between application of 90.degree. and 180.degree. pulses, is changed by a predetermined value in every encode step.
In the sequence of the SE method, in order to minimize the influences of inhomogeneity of a static magnetic field, time t=0 at which a 90.degree. pulse is applied, time t=T180 at which a 180.degree. pulse is applied, and time t=TE at which the peak of a spin echo signal appears must satisfy the following equation: EQU T180=TE/2
Preferably, the earliest timing at which echo signal acquistion can be started comes at a point A in FIG. 1 after application of a 180.degree. pulse having a pulse time Tw, at which the leading edge of the read gradient magnetic field Gr is stabilized after the trailing edge of the slicing gradient magnetic field Gs. It is well known that the echo signal acquired when the read gradient magnetic field Gr is not stabilized adversely affects the resultant magnetic resonance image. If data acquisition is to be performed in a symmetrical manner with respect to the echo peak at time t=TE, an echo signal acquisition time Taq is limited as follows: EQU TE-Tw-2.alpha..gtoreq.Taq
where .alpha. is either the fall time of a slicing gradient magnetic field Gs or the rise time of a read gradient magnetic field Gr. If the resolution remains the same, the upper limit of the time Taq is determined by TE, Tw, and .alpha.. In addition, since the strength of a gradient magnetic field cannot be much decreased and ##EQU1## the signal-to-noise (S/N) ratio cannot be increased.
As described above, according to the conventional system, a time 1/2 the echo time (the time interval between the peak of a 90.degree. pulse and the peak of an echo signal) TE is set to be a time T180, and a 180.degree. pulse is applied. That is, EQU TE/2=T180
Therefore, the upper limit of the echo signal acquisition time Taq is determined as TE-Tw-2.alpha..gtoreq.Taq, and an increase in S/N ratio is undesirably limited when the resolution and the TE time remain the same.
In recent years, however, techniques for obtaining homogeneity of a static magnetic field has progressed in MRI systems, and hence inhomogeneity of a static magnetic field can be reduced to such an extent that no problem is posed in practical use.